1. Field of the Invention
The present invention relates to a demodulator and a polarization diversity receiver for coherent optical communication provided with the demodulator.
The coherent optical transmission method is suitable for long-distance transmission because higher reception sensitivity is obtained thereby than that obtained by the intensity-modulation/direct-detection method in practical use today. Further, since frequency selection can be achieved relatively easily by electrical processing after optical detection has been made, it is suitable for transmission by frequency-division multiplexing at a high density. Now, since the term "detection" is liable to be confused with the term "demodulation", the term "detection" or "optical detection" herein will be used to mean the conversion from an optical signal to an electrical signal (intermediate-frequency signal) and the term "demodulation" will be used to mean the conversion from an intermediate-frequency signal to a baseband signal.
When signal reception is performed through heterodyne detection, it is required in mixing the signal light with the local light that the polarization state of both of the light beams is in agreement. Disagreement between the polarization states leads to deterioration in the reception sensitivity. When, for example, both the signal light and the local light are linearly polarized light and the planes of polarization are orthogonal to each other, there is produced no interference on the photodetecting surface and hence the signal reception becomes unachievable. Since single mode fibers in general are not capable of retaining the polarized state, the polarized state on the receiving end varies due to changes in the environmental conditions with time. Therefore, in order to maintain a required reception sensitivity, it becomes necessary to cope with the variation in the polarization state on the receiving end.
2. Description of the Related Art
As a technique to cope with the variation in the polarization state on the receiving end, there is a polarization diversity system. A prior art example of structure of a polarization diversity receiver for coherent optical communication to which such system is applied is shown in FIG. 19. Reference numeral 111 denotes an optical local oscillator made up of a semiconductor laser or the like. This optical local oscillator 111 outputs local light having a frequency in a specific relation with the frequency of the received signal light. Reference numeral 112 denotes an optical/electrical converter made up of polarization beam splitters, optical couplers, optical detecting circuits, and the like in combination. The converter 112 performs optical/electrical conversion of the received signal and the local light for each of the polarization components, of which the planes of polarization are orthogonal to each other, to thereby output two intermediate frequency signals (IF signals), for each of the polarization components, having the frequency corresponding to the difference between the frequency of the signal light and the frequency of the local light. One of the IF signals is input to a demodulator 115 through variable gain amplifiers 113 and 114, and the other IF signal is separately input to a demodulator 118 through variable gain amplifiers 116 and 117. Demodulated signals from the demodulators are respectively passed through variable gain amplifiers 119 and 120 and added together in an adder 121 and then input to a discriminator 122 whereby the transmitted information is regenerated. Reference numeral 123 denotes a control circuit for controlling gains in each of the variable gain amplifiers and reference numeral 124 denotes an automatic frequency control circuit for controlling the frequency of the local light so that the frequency of the IF signal may be kept constant.
The manner of the gain controlling operation performed in the control circuit 123 will be described below. Total power of the received signal light in general varies with conditions of the light transmission path and the like. Now, we represent the power of the signal light by k. Since the two IF signals output from the optical/electrical converter 112 are based on the polarization components of the signal light having planes of polarization orthogonal to each other, the ratio of power between the two IF signals becomes a: (1-a), where 0.ltoreq.a.ltoreq.1. Therefore, the power of the IF signal input from the optical/electrical converter 112 to the variable gain amplifier 113 is proportional to ka, and the power of the IF signal input from the optical/electrical converter 112 to the variable gain amplifier 116 is proportional to k(1-a). The variable gain amplifiers 113 and 116 are for coping with variation in the power of the signal light and the amplification factor of them is controlled to be proportional to k.sup.-1. The term "amplification" herein includes the case where the amplification factor is less than unity, i.e., it includes attenuation. The power of the IF signal output from the variable gain amplifier 113 is rid of the effect of the variation in the total power of the signal light and, hence, is made proportional to a and the power of the IF signal output from the variable gain amplifier 116 is rid of the effect of the same variation and, hence, is made proportional to (1-a). The variable gain amplifiers 114 and 117 are for equalizing the power of the IF signals input to the demodulators 115 and 118. Hence, the amplification factor of the variable gain amplifier 114 is controlled to be proportional to a.sup.-1 and the amplification factor of the variable gain amplifier 117 is controlled to be proportional to (1-a).sup.-1, whereby the power of the IF signals input to the demodulators 115 and 118 are made equal. The variable gain amplifiers 119 and 120 are for weighting the input signal so as to obtain a weighted sum by, for example, the maximum ratio combining law. These amplifiers 119 and 120 perform amplification of the input signals at amplification factors proportional to the S/N ratio or the signal power of the input signal. More specifically, since the power of the IF signals input to the demodulators 115 and 118 are controlled to be equal in the present example, the amplification factor of the variable gain amplifier 119 is set to be proportional to a.sup.2, while the amplification factor of the variable gain amplifier 120 is set to be proportional to (1-a).sup.2.
A prior art example of structure of the demodulator is shown in FIG. 20. This demodulator is made up of a branch circuit 130, such as a 3 dB coupler, for providing branch outputs of the input signal, a delay circuit 131 for delaying one of the output signals of the branch circuit 130 by a predetermined delay time and outputting the delayed signal, and a multiplier 132 for multiplying the other of the output signals of the branch circuit 130 and the output signals of the delay circuit 131 together.
As described above, the prior art demodulator was a combination of analog circuits and therefore it has not always been easy to form them in a monolithic integrated circuit structure.
Further, since the prior art demodulator shown in FIG. 20 is an analog circuit using a multiplier (mixer) and the like, it sometimes fails to perform a normal demodulating operation unless the level of the input to the demodulator is held constant. Because of this, in the prior art polarization diversity receiver for coherent optical communication, it was required to provide variable gain amplifiers 114 and 117, as shown in FIG. 19, for holding the power of the IF signals input to the demodulators. Thus, the circuit requires to have at least six variable gain amplifiers in all and a controlling circuit therefor. Therefore, the prior art polarization diversity receiver has had a disadvantage that its structure is complex.
In order to simplify the structure of the polarization diversity receiver, an arrangement in which a variable gain amplifier for coping with the variation in the IF signal due to the power variation in the signal light (for example, the variable gain amplifier 113) and a variable gain amplifier for controlling the power of the IF signal input to the demodulator to be constant (for example, the variable gain amplifier 114) are unified is proposed. In such case, however, the dynamic range of the gain controlling circuit for the unified variable gain amplifier is required to be higher than 30 dB, and therefore, the proposed arrangement is difficult to be practically realized.